The invention relates to a fuel pump and, in particular, a fuel pump for use in supplying fuel under high pressure to a fuel injection system.
Commonly, a common rail fuel system is used to supply fuel under high pressure to a plurality of fuel injectors for injection into the associated engine, the common rail being charged with fuel at high pressure by means an appropriate high pressure fuel pump. Typically, the pump comprises a pumping plunger which is reciprocable within a plunger bore, movement of the pumping plunger within the plunger bore being controlled by means of a cam arrangement including a cam member and a roller. The plunger bore is supplied with fuel from a low pressure pump, movement of the roller over the surface of the cam member resulting in inward movement of the pumping plunger within the bore to reduce the volume of the plunger bore, thereby increasing fuel pressure therein. The pumping plunger has an associated spring which serves to bias the pumping plunger towards its outermost position such that, on completion of inward movement of the plunger member within the bore, the pumping plunger is returned to its outermost position under the force of the spring ready for the start of the next pumping sequence.
A disadvantage of this type of pump is that a large spring force is required to bias the pumping plunger outwardly following inward movement, particularly when the engine is running at relatively high speeds. For this purpose, a large, heavy-duty spring is required. This may require the use of a pump body of relatively large dimensions and may have an impact upon the dimensions of other components of the pump, and may increase the cost of the pump.
It is an object of the present invention to provide an alternative fuel pump which can be manufactured with reduced cost.
According to the present invention, there is provided a fuel pump comprising a plunger member reciproble within a plunger bore, the plunger member being cooperable with a drive arrangement to cause inward movement of the plunger member within the plunger bore to increase fuel pressure therein, the pump further comprising an accumulator for fuel, and a valve arrangement controlling communication between the plunger bore and the accumulator to permit fuel under pressure to flow into the accumulator, and wherein the pumping plunger is moved in an outward direction under the action of the fuel pressure within the accumulator.
The invention provides the advantage that, as fuel pressure within the accumulator serves to bias the pumping plunger outwardly within the plunger bore, the need for a large and expensive spring component is removed. The pump can therefore be manufactured with reduced cost.
Conveniently, the plunger bore and the plunger member define a pumping chamber for fuel. The fuel pump may include a first valve member, for controlling communication between an inlet chamber or passage and the pumping chamber, and a second valve member for controlling communication between the pumping chamber and the accumulator. Conveniently, the first and second valve members may take the form of annular plates.
The accumulator may include an accumulator chamber, defined within an accumulator housing. The accumulator chamber may be substantially coaxially aligned with the pumping chamber. In this way, the pump can easily be formed as a single unit to minimise space.
Preferably, the accumulator housing is engageable with a seating surface defined by a seating member, the accumulator housing and the seating member being arranged such that the accumulator housing disengages the seating surface, in use, when the pressure of fuel within the accumulator chamber exceeds a predetermined amount, so as to relieve fuel pressure within the accumulator chamber.
The plunger member may be associated with a piston member, a surface of the piston member being exposed to fuel pressure within the accumulator, the force applied to the surface due to fuel pressure within the accumulator causing outward movement of the plunger member within the bore. The piston member may be integrally formed with the plunger member or may be a separate component.
Conveniently, the drive arrangement takes the form of a cam arrangement.
The plunger member is arranged to be driven in a forward direction to pressurise fuel pressure within the plunger bore. Preferably, the cam arrangement includes a cam member defining first and second cam surfaces, the first and second cam surfaces being shaped to provide a driving force to the plunger member in the forward direction for a prolonged period of time. By shaping the cam surfaces to have different forms, the period of time for which the driving force is applied to the plunger member can be increased and the driving torque can be minimised.
The accumulator may supply fuel directly to a fuel injection system, for example a plurality of fuel injection units, such that the need for a separate supply line or common rail is removed. This reduces the cost of the fuel system.